linux-kernel - Re: Disabling in-memory write cache for x86-64 in Linux II

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Message-ID: <20131031142612.GA28003@kipc2.localdomain>
Date:	Thu, 31 Oct 2013 15:26:12 +0100
From:	Karl Kiniger <karl.kiniger@....ge.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Artem S. Tashkinov" <t.artem@...os.com>,
	Wu Fengguang <fengguang.wu@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Disabling in-memory write cache for x86-64 in Linux II

On Tue 131029, Jan Kara wrote:
> On Fri 25-10-13 11:15:55, Karl Kiniger wrote:
> > On Fri 131025, Linus Torvalds wrote:
.... 
> > Is it currently possible to somehow set above values per block device?
>   Yes, to some extent. You can set /sys/block/<device>/bdi/max_ratio to
> the maximum proportion the device's dirty data can take from the total
> amount. The caveat currently is that this setting only takes effect after
> we have more than (dirty_background_ratio + dirty_ratio)/2 dirty data in
> total because that is an amount of dirty data when we start to throttle
> processes. So if the device you'd like to limit is the only one which is
> currently written to, the limiting doesn't have a big effect.

Thanks for the info - thats was I am looking for.

You are right that the limiting doesn't have a big effect right now:

on my  4x speed  DVD+RW on /dev/sr0, x86_64, 4GB,
Fedora19:

max_ratio set to 100  - about 500MB buffered, sync time 2:10 min.
max_ratio set to 1    - about 330MB buffered, sync time 1:23 min.

... way too much buffering.

(measured with strace -tt -ewrite dd if=/dev/zero of=bigfile bs=1M count=1000
by looking at the timestamps).


Karl

....
								Honza
> -- 
> Jan Kara <jack@...e.cz>
> SUSE Labs, CR

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